The unimolecular decomposition of (CH3)(2)COO and (CD3)(2)COO was measured by direct detection of the Criegee intermediate at temperatures from 283 to 323 K using time-resolved UV absorption spectroscopy. The unimolecular rate coefficient k(d) for (CH3)(2)COO shows a strong temperature dependence, increasing from 269 +/- 82 s(-1) at 283 K to 916 +/- 56 s(-1) at 323 K with an Arrhenius activation energy of similar to 6 kcal mol(-1). The bimolecular rate coefficient for the reaction of (CH3)(2)COO with SO2, k(SO2), was also determined in the temperature range 283 to 303 K. Our temperature-dependent values for k(d) and k(SO2) are consistent with previously reported relative rate coefficients k(d)/k(SO2) of (CH3)(2)COO formed from ozonolysis of tetramethyl ethylene. Quantum chemical calculations of kd for (CH3)(2)COO are consistent with the experiment, and the combination of experiment and theory for (CD3)(2)COO indicates that tunneling plays a significant role in (CH3)(2)COO unimolecular decomposition. The fast rates of unimolecular decomposition for (CH3)(2)COO measured here, in light of the relatively slow rate for the reaction of (CH3)(2)COO with water previously reported, suggest that thermal decomposition may compete with the reactions with water and with SO2 for atmospheric removal of the dimethyl-substituted Criegee intermediate.